GeezerVison is showing github.com — Possible data race in `_io_IncrementalNewlineDecoder_reset_impl` in `textio.c` · Issue #144777 · python/cpython
Possible data race in _io_IncrementalNewlineDecoder_reset_impl in textio.c #144777DescriptionBug report
Bug description:
This code produces TSAN warning on no-gil build, but I'm not sure if this is a real bug since the docs mention TextIOWrapper isn't thread safe.
import codecs
from threading import Thread
from io import IncrementalNewlineDecoder
utf8_decoder = codecs.getincrementaldecoder('utf-8')()
OBJ = IncrementalNewlineDecoder(utf8_decoder, translate=True)
def test1():
for _ in range(100):
OBJ.reset()
def main():
threads = []
for _ in range(4):
threads.append(Thread(target=test1))
for t in threads:
t.start()
for t in threads:
t.join()
if __name__ == "__main__":
main()TSAN report:
WARNING: ThreadSanitizer: data race (pid=481349)
Write of size 1 at 0x7fffb496d230 by thread T4:
#0 _io_IncrementalNewlineDecoder_reset_impl /cpython/./Modules/_io/textio.c:627:21 (python3.15t+0x5b8b15)
#1 _io_IncrementalNewlineDecoder_reset /cpython/./Modules/_io/clinic/textio.c.h:492:12 (python3.15t+0x5b8b15)
#2 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:3943:35 (python3.15t+0x42d408)
#3 _PyEval_EvalFrame /cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15t+0x422090)
#4 _PyEval_Vector /cpython/Python/ceval.c:2125:12 (python3.15t+0x422090)
#5 _PyFunction_Vectorcall /cpython/Objects/call.c (python3.15t+0x20cc0f)
#6 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x2112e6)
#7 method_vectorcall /cpython/Objects/classobject.c:74:20 (python3.15t+0x2112e6)
#8 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x476281)
#9 context_run /cpython/Python/context.c:727:29 (python3.15t+0x476281)
#10 _PyCallMethodDescriptorFastWithKeywords_StackRefSteal /cpython/Python/ceval.c:913:11 (python3.15t+0x423f4c)
#11 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:3852:35 (python3.15t+0x42d1e9)
#12 _PyEval_EvalFrame /cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15t+0x422090)
#13 _PyEval_Vector /cpython/Python/ceval.c:2125:12 (python3.15t+0x422090)
#14 _PyFunction_Vectorcall /cpython/Objects/call.c (python3.15t+0x20cc0f)
#15 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x2112e6)
#16 method_vectorcall /cpython/Objects/classobject.c:74:20 (python3.15t+0x2112e6)
#17 _PyVectorcall_Call /cpython/Objects/call.c:273:16 (python3.15t+0x20c89b)
#18 _PyObject_Call /cpython/Objects/call.c:348:16 (python3.15t+0x20c89b)
#19 PyObject_Call /cpython/Objects/call.c:373:12 (python3.15t+0x20c905)
#20 thread_run /cpython/./Modules/_threadmodule.c:387:21 (python3.15t+0x5f5512)
#21 pythread_wrapper /cpython/Python/thread_pthread.h:234:5 (python3.15t+0x526127)
Previous write of size 1 at 0x7fffb496d230 by thread T3:
#0 _io_IncrementalNewlineDecoder_reset_impl /cpython/./Modules/_io/textio.c:627:21 (python3.15t+0x5b8b15)
#1 _io_IncrementalNewlineDecoder_reset /cpython/./Modules/_io/clinic/textio.c.h:492:12 (python3.15t+0x5b8b15)
#2 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:3943:35 (python3.15t+0x42d408)
#3 _PyEval_EvalFrame /cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15t+0x422090)
#4 _PyEval_Vector /cpython/Python/ceval.c:2125:12 (python3.15t+0x422090)
#5 _PyFunction_Vectorcall /cpython/Objects/call.c (python3.15t+0x20cc0f)
#6 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x2112e6)
#7 method_vectorcall /cpython/Objects/classobject.c:74:20 (python3.15t+0x2112e6)
#8 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x476281)
#9 context_run /cpython/Python/context.c:727:29 (python3.15t+0x476281)
#10 _PyCallMethodDescriptorFastWithKeywords_StackRefSteal /cpython/Python/ceval.c:913:11 (python3.15t+0x423f4c)
#11 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:3852:35 (python3.15t+0x42d1e9)
#12 _PyEval_EvalFrame /cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15t+0x422090)
#13 _PyEval_Vector /cpython/Python/ceval.c:2125:12 (python3.15t+0x422090)
#14 _PyFunction_Vectorcall /cpython/Objects/call.c (python3.15t+0x20cc0f)
#15 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x2112e6)
#16 method_vectorcall /cpython/Objects/classobject.c:74:20 (python3.15t+0x2112e6)
#17 _PyVectorcall_Call /cpython/Objects/call.c:273:16 (python3.15t+0x20c89b)
#18 _PyObject_Call /cpython/Objects/call.c:348:16 (python3.15t+0x20c89b)
#19 PyObject_Call /cpython/Objects/call.c:373:12 (python3.15t+0x20c905)
#20 thread_run /cpython/./Modules/_threadmodule.c:387:21 (python3.15t+0x5f5512)
#21 pythread_wrapper /cpython/Python/thread_pthread.h:234:5 (python3.15t+0x526127)
SUMMARY: ThreadSanitizer: data race /cpython/./Modules/_io/textio.c:627:21 in _io_IncrementalNewlineDecoder_reset_impl
Version: Python 3.15.0a5+ free-threading build (heads/main:d5f96c86653, Feb 11 2026, 04:50:14) [Clang 18.1.3 (1ubuntu1)]
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux
Linked PRs
ActivityKrishna-web-hub commented on Feb 13, 2026 Hello @Naserume , i think this error is due to the free threaded python which causes data race condition and also in the doc it is mentioned that TextIOWrapper is not safe. I think @StanFromIreland can give us some direction on whether to work on it or not. colesbury commented on Feb 23, 2026 Thanks for the bug report!
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